In the field of the prosthesis, the application of the artificial hipjoint is the most difficult task because the hipjoint must not only bear a load but also exercise as a function of bearing. Therefore, the material of the artificial hipjoint must have high strength and high toughness. On the other hand, the contact surface of an artificial hipjoint must also have high wearability for the daily wear.
The osteolysis is the most serious problem in the application of the artificial skeleton. The metal-polymer composite is commonly used in prosthesis, but the wear of the polymer used for the artificial hipjoint results in a generation of the scraps which will be absorbed by the human body. Therefore, the metal-polymer artificial hipjoint, implanted in a human body, has to be replaced every few years. The biomedical material engineers find out that the ceramic with its chemical stability, high wearability, and high strength is a good material to replace the polymer part of the composite. By minimizing the scraps, the osteolysis is lessened and the use of the metal-ceramic composite in the human body has already obtained a good clinic result.
The usage of alumina in the artificial skeleton is not a new subject (U.S. Pat. Nos. 3,871,031 and 3,905,047). The ISO 6474:1994(E) has defined the physical and chemical properties of the alumina material in detail, such as the composition, design, mechanical strength, and testing methods, and many papers have been published in this field. If the alumina prosthesis can consist with the standards, it is allowed to be used in a human body clinically.
The alumina powder used for the prosthesis has to be very fine (submicro) and pure (&gt;99.8%) for the green compact after sintering to have a homogeneously fine grain structure without large internal defects (&gt;5 .mu.m) to obtain high strength. The conventional process for producing the alumina material is powder metallurgy. However, the cost for producing an alumina skeleton by powder metallurgy is high because of the complex processes, including the cold isotropic pressing, hot isotropic pressing, and mechanical polishing. The technique for producing an alumina skeleton is difficult, too. It is hard to compact the fine powder by conventional compacting methods to obtain a high density because of the agglomeration and low flowability of fine powders. The material produced by this method does not have good mechanical property and microstructure. (L. L. Hench and D. R. Ullrich, "Ultrastructure Processing of Ceramics, Glass, and Composites", John Wiley & Sons, pp. 407-417, 1984)
The colloidal process is another process for producing the artificial skeleton. It uses a slurry to form the green part. By adding a dispersing agent to the slurry and controlling its pH value, the electrostatic repulsion and steric hindrance make the particles in the slurry have a good dispersion, even submicro particles, and the green part can have a high strength (J. Cesarno III, and I. A. Aksay, J. Am. Ceram. Soc., 71(4), pp. 250-255, 1998). Although the colloidal process is a good process for producing a fine powder ceramic material, there are many factors affecting the process as follows:
(1) The Surface Potential:
The ceramic powder has a large specific surface area and a poor solubility and, therefore, the powder is easily induced to have a charge. The negative particles attract positive particles and repel particles with same charges because of the Coulombic force. Although the slurry is neutral in electricity, there are potential differences on the particles. If an electric field is applied to the slurry, the particles will move electrophoretically and a part of the slurry adsorbed by the particles will also move. (J. S. Reed, Introduction to the Principles of Ceramics Processing, John Wiley & Sons, 1988)
(2) The pH Value of the Slurry:
The pH value of the slurry can affect the surface charges of the particles. The degree of ionization is changed with the pH value, and the ionization makes the electric steric effect between the alumina particles more serious.
(3) The Dispersing Agent:
The polymethacrylic acid (PMAA) and polyacrylic acid (PAA) are the most popular and effective dispersing agents used in the alumina slurry. The dispersing agents cause the particles in the slurry to have a better dispersion because of the large electrosteric effect. (J. Cesarno III, and I. A. Aksay, J. Am. Ceram. Soc., 71(12), pp. 1062-1067, 1988)
(4) The Flowing Behavior:
The viscosity of the slurry is a critical factor in its flowability. If the viscosity of the slurry is too low, the sedimentation of the particles will induce the particles to segregate. If the viscosity is too high, the bubbles in the slurry will not come out easily. (B. V. Velamakanni and F. F. Lange, J. Am. Ceram. Soc., 74(1), pp. 166-172, 1991)
(5) The Shape of the Particles:
When the aspect ratio of the powder increases, the green density will decrease. The material made by the powder with the aspect ratio equals to one (equi-axial) will have the highest green density.
(6) The Particle Size Distribution:
By carefully controlling the particle size distribution of the powder, the small particles can fill in the interspace of the large particles and the green density can be effectively raised.
Because the colloidal process can effectively mix particles with different sizes to produce a uniform ceramic green part, the sintering density of the green part is increased. There has been reported that the .alpha.-alumina can improve the phase transformation of the boehmite and the .theta.-alumina during the sintering process. By adding a little amount of the .alpha.-alumina to the .theta.-alumina slurry, the sintering rate and the sintering density are increased. Although the reason is still unknown, this discovery is very important for controlling the sintering rate and the microstructure of the .theta.-alumina (M. Kumagai and G. L. Messing, "Enhanced densification of boehmite colloidals by .alpha.-alumina seeding", Comm. Am. Ceram. Soc., C230-31, 1984).
The main concern of the present invention is to provide a process for producing an alumina material with high strength which is suitable to be used in the artificial skeleton.